NIAR Explores Advances In Composites Technology

When Wichita thought of tearing down a former concert and sports arena on the north end of town, John Tomblin, executive director of the National Institute for Aviation Research (NIAR), told city leaders there was a good reason to save the 56,000-sq.-ft. structure. Its domed roofline and big flat floor were just what the institute needed for an aircraft test center. So out came the concert seats and in went a hangar door.

When the conversion was completed last February, NIAR already had its first client. Learjet, one of the institute's early backers among Wichita's diverse aviation manufacturing community, brought in the static test article for its first composite aircraft, the Learjet 85. Composites are one of NIAR's main research subjects and the new 8-place mid-sized business jet offers an early chance to test the fatigue life of an unusual set of wings.

Learjet and its parent, Canada's Bombardier Aerospace, have developed a proprietary resin transfer injection (RTI) technology aimed at eliminating the need to pre-impregnate (prepreg) carbon fiber reinforced plastic (CFRP) before the fabric is wound into or laid up in the build process. Prepreg is the industry's dominant method for creating composite parts and assemblies, especially large structures, but it is time-consuming and expensive.

In RTI, the carbon fiber is laid up dry. The resin that will bind the multiple fabric layers that make up a composite structure is not injected until the completed structure is in the autoclave. Although the method has not been applied universally, it works particularly well on flat surfaces that can be formed horizontally, such as wings and spars, and it allows skins and stringers to be formed in a single piece.

“Prepreg is clock-limited because the resin cures, so it has to be stored properly,” he says. When fabric is resin-infused it has to be stored cold because it begins curing in warm air. Because the dry-fabric RTI process does not receive its epoxy infusion until curing is ready to begin, it automatically assures greater flexibility in the manufacturing process, Harter says.

But Bombardier's patent-pending process, which the company describes only in general terms, does not work on every large structure, such as fuselage barrel formation because gravity prevents the non-adhesive dry tapes from staying in place on the undersides of structures. The Learjet 85 has a 32-ft.-long carbon fiber barrel section produced in the company's Queretaro, Mexico, factory, but it is created with traditional pre-preg, not RTI.

The company also does not apply the process for the vertical and horizontal stabilizers, Harter says, because they do not have integral stringers.

Even so, Tomblin sees great potential from a quality standpoint for the process. “Resin systems are so advanced that they can approach autoclave quality with near-zero voids,” he says.

Bombardier's RTI process has been in test for several years. It took eight months to construct the Learjet 85 test article.

Like so many others, Bombardier is following in Boeing's footsteps with regard to introducing large-scale composite structures into commercial aviation manufacturing. Boeing began experimenting with prepreg composites in the late 1970s with 10 737-200 horizontal stabilizers that were put into commercial service for about 15 years. When those aircraft were retired, Boeing and NIAR did a post-op examination of the tails and could find no evidence of fatigue. “They don't age,” Tomblin says of the composite structures, which is exactly what Boeing says when it markets the 787.

From there, use of CFRP progressed through the 767 to the 777, where it first emerged in a large flight-critical structure on the rudder. Again, these showed excellent service-life qualities. The 777 was the warm-up for Boeing's real goal, shifting entirely away from aluminum alloy airframes on the 787. For that aircraft, complete fuselage barrels and wings are produced with prepreg carbon fiber tape cured in autoclaves.

The autoclaves needed to cure composite structures are expensive to buy and operate, given that cure times for large structures like a wingbox or fuselage barrel are measured in shifts, not minutes, and tie up the entire machine.

So the industry is looking for ways to avoid autoclaves. Many of the 787's smaller composite parts are not dependent on autoclaves, such as the vacuum-assisted resin-transfer molding process used by Hawker de Havilland, a Boeing subsidiary, for trailing edges. One assignment for the company's Fabrication Div. is to explore ways to eliminate autoclaves.

Much of NIAR's research help for its industrial clients is process-oriented, tackling “what-if” projects to discover how well they will work in manufacturing to produce a flyable product.

Although it is a practical, not academic, institution, NIAR's home is on the campus of Wichita State University. “We are geared around certification,” Tomblin, its founder and executive director, explains. “Getting a part onto a plane is what we do.”

The institute has 180 clients, and a $50 million annual budget, half of which comes from commercial customers. In fact, it was Learjet, Beechcraft, Cessna and Boeing Wichita (now Spirit AeroSystems) that provided Tomblin with the industrial backing to open NIAR's doors in 1985. It is the third largest aerospace center in the U.S., and easily the largest focused on aircraft research that directly engages industry and is not heavily dependent on federal funding, he says.

Its staff of 350 includes WSU undergraduates and graduate students headed for aerospace careers. Harter is among the alumni.

The institute operates two FAA Centers of Excellence, has five locations encompassing 320,000 sq. ft. of laboratory and office space, and outsources its skills. For instance, it runs test labs for Beech. “We don't all need to own a wind tunnel. We don't all need to own a crash sled,” Tomblin says, noting that even though Wichita's aviation industrial base is a ready source for projects, NIAR has a global client list.

NIAR also specializes in figuring out how to keep aircraft flying, given foreign object damage, clumsy airfield workers, lightning strikes and the fatigue of thousands of takeoff and landing cycles that can play havoc with an airframe.

Where ice hockey teams used to suit up, NIAR now houses a machine shop largely devoted to aging-aircraft studies. Its star is a 500,000-lb. compressor that can apply 60,000 ft.-lb. of torsion.

With it and smaller machines, NIAR can measure materials capability over full lifecycles. One contract is a “keep-'em-flying” contract for U.S. Air Force KC-135Es. “We're looking for corrosion and fatigue in places that have never been opened up,” Tomblin says.

The shop also tests the strength of structural repairs that manufacturers undertake to save an improperly made structure, such as a void.

Back at Wichita State, the National Center for Advanced Materials Performance is the largest private test center certified by the FAA, banging, zapping, puncturing and otherwise abusing about 1,000 test coupons a week.

The center establishes procedures and protocols for manufacturing testing that are the equivalent of materials specifications. An analogy is the industry standard 20214T3 aluminum specification that has its roots in the material used in World War II. “We want a testing regime to be of a similar spec,” Tomblin says. Composites developed in the early- and mid-1990s that are now common can be characterized by material and specification. Procedures for manufacturing with them can be certified by an equivalency process.

“Changing materials is a big deal” because of the time and testing required for certification, Tomblin says. For that reason, Lockheed Martin's F-22 was built using 25-year-old material and the F-35 uses the same material, he says. The composite material from the 777's empennage was accepted for the 787.

NCAM is trying to break down certification and time elements so baseline performance levels of new materials can be established quicker. Their goal is to drop them by 1/6th, meaning a three-year schedule to achieve certification can be dropped by six months.

One of NIAR's research efforts is on improving lightning protection systems for composite skins. Boeing's method is proprietary, but involves an interweave of conductive wires in the outermost plies of the fuselage. For the B-2 bomber, the protection system was actually part of the radar imaging-resistant coating. If the conductive wires are buried any deeper than the outer layer they will be ineffective, says NIAR senior research engineer Lamia Salah, who leads its advanced materials laboratory.

NIAR is working on electromagnetic interference-resistant nano skins that can be applied directly onto composite structures as part of the RTI process. The skins would provide lightning protection and serve other purposes for military aircraft.

Such experiments hold promise for reducing composite manufacturing costs, says Tomblin. Besides the cost of buying the lightning-protective mesh itself, there is the cost of integrating it into a structure, he says. Applying a nano skin as just another layer in a resin transfer molding takes out this middle step.